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Ova and Parasite Exam
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Este artigo foi modificado pela última vez em 10 de Julho de 2017.
At a Glance
Why Get Tested?
To determine whether you have a parasite infecting your gastrointestinal tract
When To Get Tested?
When you have diarrhea that lasts more than a few days and/or have blood or mucus in your loose stools, especially if you have drunk unpurified water while camping or have traveled outside of the United States
Sample Required?
A fresh or preserved stool sample, sometimes multiple samples collected on different days
What is being tested?
An ova and parasite (O&P) exam is a microscopic evaluation that is used to look for parasites that have infected the gastrointestinal tract. The parasites are shed from the gastrointestinal tract into the feces. When thin smears of fresh or preserved stool are put onto glass slides and stained, the parasites and their ova or cyst (the form in which the parasite is surrounded by a resistant cover or capsule) can be detected and identified under the microscope. Different ova and parasites have distinct shapes, sizes, and internal structures that are characteristic of their species.

There are a wide variety of parasites that can infect humans. Each type of parasite has a specific life cycle and maturation process and may live in one or more hosts. Some parasites spend part of their life in an intermediate host, such as a sheep, cow, or snail, before infecting humans. Some infect humans “by accident.” Most parasites have an adult form and a cyst/egg/ova form. Some also mature through a larval phase, which is between the egg and the adult. Ova are hardy and can exist for some time in the environment without a host.

The majority of people who are infected by gastrointestinal parasites become infected by drinking water or eating food that has been contaminated with the ova. This contamination cannot be seen; the food and water will look, smell, and taste completely normal. Since an infected person’s stool will frequently contain ova, it is also a source of infection. Without careful sanitation (handwashing and care with food preparation), the infection may be passed on to others. This is especially a concern with infants at day care centers and the elderly in nursing homes. In these populations, a parasitic infection may be easily spread, and the immune systems of those infected may be less effective at getting rid of the infection.

The three most common parasites in the United States are single cell parasites: Giardia lamblia, Entamoeba histolytica (E. histolytica), and Cryptosporidium parvum. The Centers for Disease Control and Prevention (CDC) estimate that there are about 2 million giardia infections in the United States each year and that cryptosporidium (commonly called crypto) is the most common cause of recreational water-related disease outbreaks. Found in mountain streams and lakes throughout the world, these parasites may infect swimming pools, hot tubs, and occasionally community water supplies. Cryptosporidia resist chlorine and can live for several days in swimming pools. In the U.S., most parasitic infections are due to these three, but other parasites such as roundworms or tapeworms do occasionally cause infections.

Those who travel outside the U.S., especially to developing nations, may be exposed to a much wider variety of parasites. In warm climates and places where water and sewage treatment are less effective, parasites are more prevalent. Besides giardia, crypto, and E. histolytica, there are also a wide range of flat worms, roundworms, hookworms, and flukes that can affect the gastrointestinal tract and other parts of the body. Visitors usually become infected by eating or drinking something that has been contaminated with the parasites’ ova - even something as simple as ice cubes in a drink or a fresh salad - but some of the parasites can also penetrate the skin, such as through the skin of the foot when someone is walking barefoot.

The most common symptoms of a parasitic infection are prolonged diarrhea, bloody diarrhea, mucus in stool, abdominal pain, and nausea. Patients may also have headaches and fever or few or no noticeable symptoms.

How is the sample collected for testing?

A fresh stool sample is collected in a clean container. The stool sample should not be contaminated with urine or water. Once it has been collected, the stool should either be taken to the laboratory within an hour after collection or transferred into special transport vials containing preservative solutions.

Once the stool has been preserved, it should be brought to the laboratory as soon as possible for examination. If your doctor has ordered multiple samples, you may collect and transfer all of the stool samples into the preservative vials before returning them to the laboratory. When multiple samples are ordered, they should be collected at different times on different days because parasites are shed intermittently and may not be in the stool at all times. Each vial should be labeled with the patient’s name and the date and time of the stool collection.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.
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Common Questions
  • How is it used?
    The O&P exam is used to diagnose the cause of prolonged diarrhea. It is ordered to determine whether there are parasites present in the gastrointestinal tract and, if so, to identify them. Since there are many other causes of diarrhea, the O&P is often ordered along with other tests, such as a stool culture, which identifies the presence of pathogenic bacteria in the stool. Classically, O&P exams were ordered in multiples, such as three samples from three different bowel movements, often on separate days. This was considered to be the best way to detect what might be small amounts of ova in the stool, providing a better chance of not missing an infection. Your doctor may order a giardia, cryptosporidium, or E. histolytica antigen test if he suspects that one of these parasites may be causing your diarrhea. These tests detect protein structures on the parasites and can identify an infection, even if no actual parasites or ova are seen in the stool. Since antigen tests only detect a few specific parasites, they are not replacements for the complete O&P, which will detect a wider variety of parasites.

    O&P tests may also be ordered to monitor the effectiveness of treatment for a parasitic infection.

  • When is it ordered?
    When you have prolonged diarrhea, abdominal pain, and/or blood and mucus in your stool. When you have symptoms and have recently drunk stream or lake water while camping, been exposed to someone who has a parasitic infection (like a family member), or traveled outside the U.S.

    O&P tests may also be ordered when you have had a parasitic infection, to monitor the effectiveness of treatment.

  • What does the test result mean?
    If there are no ova or parasites seen, your diarrhea may be due to another cause. There may also be too few parasites to detect. Your doctor may want to order additional O&P tests and/or may order other tests to search for the cause of your symptoms.

    If a parasite is identified, then you do have a parasitic infection. The type and duration of treatment will depend on what kinds of parasite(s) are found and on your general state of health. The number of parasites seen may give your doctor general information about how heavy or extensive your infection is.

  • Is there anything else I should know?
    If diarrhea lasts more than a few days, it may lead to dehydration and electrolyte imbalance, dangerous conditions in children and the elderly.

    Drug therapies are usually used to treat giardia and E. histolytica infections. They may resolve themselves after several weeks, but they may also cycle, with symptoms subsiding and then worsening again. There is no effective treatment for cryptosporidium. In those with competent immune systems, crypto generally goes away after a few weeks. In those with compromised immune systems (such as those with HIV/AIDS, organ transplant, cancer, etc.), however, crypto may be dangerous, becoming chronic and causing wasting and malnutrition.

    Parasitic infections are monitored on a community level. Other than travel-related cases, health officials want to try to determine where your infection came from so that they can address any potential public health concerns. For instance, if crypto or giardia is due to contaminated swimming pool water or community water supply, steps will need to be taken to prevent the spread of the infection. Parasites can be eliminated from the water supply with chlorine treatment and/or with adequate filtration.

  • How can I prevent a parasitic infection?
    The best way is to avoid food and water that is suspected of being contaminated. This is especially true if you travel to developing nations, where ice in a drink or a dinner salad may expose you to parasites. But the clearest mountain stream should also be suspect; it could be contaminated with giardia. You cannot see most parasites; you won’t be able to smell them or taste them in the water. If someone in your family has a parasitic infection, careful handwashing after going to the bathroom can help prevent passing the parasite on to others. The infected person should avoid preparing food for others until their symptoms are gone.
  • Will an O&P detect all parasites?
    No, only those that live in the intestines and whose eggs are passed through the feces. There are other tests specific for other parasites, such as pinworms or blood parasites that cause malaria.
  • Why does it need to be a fresh stool sample?
    The structure of the parasites may deteriorate in unpreserved stool, which would destroy the identifying characteristics of the parasite and make the infection harder to detect.
  • Are those parasitic worms I’m seeing in my stool sample?
    Most of the common parasites are much too small to be seen with the naked eye. What you are probably noticing are undigested food fibers. The only way to be sure though is to look at the sample under the microscope. Parasites have characteristic external and internal structures that fibers do not have.
  • Are there other ways to test for parasites besides the microscopic exam?
    Yes, antigen tests have been developed for several common parasites including giardia, cryptosporidium, and E. histolytica. The antigen tests detect protein structures on the parasite, and they can detect the presence of fragments of the parasite in a stool sample. This has the advantage of allowing detection of that particular parasite even if it is not seen in the microscopic O&P examination. Blood antibody tests may be ordered to determine whether or not someone has been exposed to a parasite in the past. This may indicate a past or a chronic infection but is not used to detect a current infection. Sometimes a biopsy of the small intestines is taken, and the small amount of tissue is examined for parasitic infestation.
  • Once I’ve had a parasitic infection, can I be re-infected?
    Yes, you can become re-infected if exposed again. This may happen if you have a family member who has an asymptomatic parasitic infection, such as giardiasis (due to giardia), and continues to shed the organisms and re-infects others until everyone is treated.
  • Why shouldn’t I take an over the counter anti-diarrhea medicine?
    You should only take this on the advice of your doctor. Diarrhea is one of the methods your body uses to help rid itself of the infection. If you slow down or prevent this from happening by taking anti-diarrhea medication, you can prolong the amount of time that you are ill and may make your infection worse.
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